As I brought my car out for the season, I noticed on WOT that I am getting an intermittent misfire/stumble....roughly around 4000rpm in second or third gear. What I mean by intermittent I mean once or twice in a 30 mile run. The engine light tends to come on and off but have always thought that is the air fuel sensor inherent issue all leans have.Checking for codes got 33, 41, 42, and 54.I cleared them and rechecked and all was clear. I then ran the car at idle instantly getting an engine warning lamp.So rechecked for codes and got 41.I recently had the head skimmed and a new cam belt fitted (by Paul Matty sports cars) and have done around 500 years.Have checked around the edge bay for anything obvious but all looks good.(note my car is a 92 SE without an O2 sensor)I have seen a few threads on chipping the ECU, but have no idea how to check for this, but as far as I know it isn't chipped, plus others have mentioned plug gaps, running higher octane fuel, or even cracked spark plugs.Any thoughts on where I should go next?

Simon, thanks for the info.Have checked earthing and plugs, all looks good the the leads are old.Have reset the ecu, filled it with high octane fuel this morning and took it for a drive and code 41 is still there.In fact the engine light is now tending to be on more times than not.Checked the Hall sensor and it is the old one, so looking like that's the culprit.Will let you know when it's been swapped outThanks for your help Carl

Finally got the sensor replaced and she is running like a dream! Though broke 2 of the 3 bolts when trying to remove the CAS cap. Bugger! After 25 years they had welded themselves to the aluminium casting!Thanks for all the help resolving the problem!